The present invention relates generally to analysis of substrates which are coated, and more particularly to a system and method for imaging the surface of a substrate which is coated through the coating for the purposes of detecting rust, pitting, corrosion, cracks, scratches, gouges, and other structural imperfections.
Aircraft components are subject to constant degradation caused by environmental conditions. Various agents including moisture, dust, wind, solar radiation, and air pollutants cause damage to components in the form of rust or corrosion. Although the application of a coating, such as paint, reduces these problems substantially, it typically cannot eliminate them entirely. Moreover, other causes such as stress experienced during flight can result in damage which a coating of paint cannot mitigate, such as stress defects and cracking. While the occurrence of these forms of damage is to be expected, the particular rate at which any given aircraft's components degrade is highly dependent upon the particular environment of the aircraft and the circumstances under which it operates. This is readily apparent at aircraft maintenance depots, where maintenance personnel sometimes have the opportunity to view two aircraft of similar make and age. In many instances, the need for repair or replacement of components is much greater for one such aircraft than for the other. It is therefore impractical to rely upon projected maintenance schedules in determining when an aircraft will need repair. The only effective way to ensure that aircraft are ready for flight is through periodic inspection.
Using traditional methods, inspection of aircraft components is accomplished by means of visual inspection. When visually inspecting aircraft components, the coating used to protect the components becomes an obstacle because it may hide structural defects beneath. It is therefore necessary to strip the component assembly or aircraft in question of its paint before a proper inspection can be performed. Afterward, a new coating of paint must be applied. Obviously, this process results in substantial expense in the form of labor and materials, and likewise requires a great amount of time. It has been estimated that an aircraft spends twelve percent of its life in some form of maintenance or inspection, and billions of dollars are spent on aircraft maintenance every year. Apart from the inefficiency of visual inspection methods, another problem is the fact that visual inspection is simply not as effective as might be desired. While a skillful eye may pick up most human-visible defects with a satisfactory degree of consistency, some defects may be very small or lie under the surface of the component. In many cases these defects will go unnoticed by the naked human eye, regardless of the skill and experience of the observer. It is therefore desirable to devise a method for analyzing damage to aircraft components without the need to strip paint from the component or rely upon the human eye alone. Some inventions offer insight into how this problem might be solved.
One such invention is described in U.S. Pat. No. 5,426,506 entitled OPTICAL METHOD AND APPARATUS FOR DETECTION OF SURFACE AND NEAR-SUBSURFACE DEFECTS IN DENSE CERAMICS issued to Ellingson, et al. The invention described therein employs a laser of a wavelength calculated to penetrate the surface of an object to be analyzed. The laser is passed through a polarizer before being reflected by the object, and through a second polarizer afterward. When striking the object, that portion of light which strikes irregularities is reflected at an altered polarity, while the portion which strikes regular features is reflected at its original polarity. The second polarizer is configured to detect this difference, and the system generates an image reflecting it. In order to generate an image of an area, the object to be analyzed is secured to a mount capable of translation and/or rotation and controlled by a computer or similar device. The object is moved about under the laser beam, to thereby be scanned. The most obvious disadvantage of this system is that in order to perform area analysis, a motorized mount typically must be used. This appears to preclude the possibility of a hand-held unit, and the system would be highly impractical when applied to components or assemblies already mounted on aircraft. Another obvious disadvantage is the method is not used to see surfaces under organic coatings and the wavelength of the laser light will not penetrate coatings or polymers.
A second related invention is disclosed in U.S. Pat. No. 4,682,222 entitled STIMULATED SCANNING INFRARED SYSTEM issued to Smith et al. The invention uses a collimated energy beam, such as a laser, to heat an object to be analyzed. Because objects radiate infrared light when they are warm, an infrared detector can then be used to detect the heat of areas of the object relative to each other. For instance, because areas which are cracked will heat at a different rate than other areas, they can thereby be distinguished. The obvious disadvantage of this system is that the object to be scanned must be heated. For various reasons, methods involving heating the object to be analyzed are not ideal. For instance, a thermal shielding component of an aircraft with a coating of paint would pose a particular problem for this system. The component is specifically designed to be difficult to heat, and any source powerful enough to heat the component would likely damage the coating of paint. This patent additionally utilizes a technique of thermography which does not relate to IR imaging of substrate surfaces under organic coatings.
Still another related invention is disclosed in U.S. Pat. No. 6,184,528 entitled METHOD OF SPECTRAL NONDESTRUCTIVE EVALUATION issued to DiMarzio, et al. The invention disclosed therein employs an infrared light source, such as an infrared laser, to cast infrared light upon a substrate. Reflected light is measured as a function of wavelength to obtain reflectivity data. The reflectivity data of the sample substrate is compared to reflectivity data of a control substrate. Correlations are then drawn between differences in order to determine the presence of corrosion. This invention achieves some of the objectives of the present invention, but will not detect the full range of structural features detectable by the present invention and does not provide a visual image of the substrate.
It is therefore desirable to devise a system and method for analyzing substrates free of the aforementioned drawbacks and, further, improving upon previous systems in terms of effectiveness and resolution.